|KNAUBER, DONNA - Retired ARS Employee|
Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/18/2013
Publication Date: 2/3/2014
Citation: Suttle, J.C., Huckle, L.L., Lu, S., Knauber, D.C. 2014. Potato tuber cytokinin oxidase/dehydrogenase genes: Biochemical properties, activity, and expression during tuber dormancy progression. Journal of Plant Physiology. 171:448-457.
Interpretive Summary: For an indeterminate period of time following harvest, potatoes will not sprout and are physiologically dormant. Dormancy is gradually lost during postharvest storage and the resultant sprouting is detrimental to the nutritional and processing qualities of potatoes. Because of this, sprouting results in severe financial loss to producers. Currently sprouting is controlled through the use of synthetic sprout inhibitors. The research being conducted in this project is directed towards: 1.) identifying key physiological processes that naturally regulate tuber dormancy and, ultimately, 2.) modifying these processes genetically thereby eliminating the need for artificial sprout suppression. Previous research from this laboratory has demonstrated that dormancy exit and the onset of sprout growth coincides with and is likely dependent on an increase in cytokinin content (hormones required for cell division). In many plants, cytokinin content is regulated in part by the activity of the enzyme cytokinin oxidase/dehydrogenase (CKX) which metabolizes cytokinins into inactive products. In this paper, the biochemical and enzymatic properties of five potato tuber genes that encode CKX-like proteins were determined and the role of CKX in the control of potato tuber dormancy was examined by determining the rates of cytokinin metabolism and CKX gene expression during natural and chemically forced dormancy progression. All five CKX-like genes were found to encode proteins that exhibited CKX enzymatic activity but differed slightly in other biochemical properties. The rate of cytokinin metabolism and the expression of the five CKX genes did not change significantly during either natural or chemically forced dormancy progression. These results indicate that cytokinin metabolism and CKX gene expression do not play major roles in the regulation of potato tuber dormancy.
Technical Abstract: The enzymatic and biochemical properties of the proteins encoded by five potato cytokinin oxidase/dehydrogenase (CKX)-like genes functionally expressed in yeast and the effects of tuber dormancy progression on StCKX expression and cytokinin metabolism were examined in meristems isolated from field-grown tubers. All five putative StCKX genes encoded proteins with in vitro CKX activity. All five enzymes were maximally active at neutral to slightly alkaline pH with 2,6-dichloro-indophenol as the electron acceptor. In silico analyses indicated that four proteins were likely secreted. Substrate dependence of two of the most active enzymes varied; one exhibiting greater activity with isopentenyl-type cytokinins while the other was maximally active with cis-zeatin as a substrate. [3H]-isopentenyl-adenosine was readily metabolized by excised tuber meristems to adenine/adenosine demonstrating that CKX was active in planta. There was no change in apparent in planta CKX activity during either natural or chemically forced dormancy progression. Similarly although expression of individual StCKX genes varied modestly during tuber dormancy, there was no clear correlation between StCKX gene expression and tuber dormancy status. Thus although CKX gene expression and enzyme activity are present in potato tuber meristems throughout dormancy, they do not appear to play a significant role in the regulation of cytokinin content during tuber dormancy progression.